1. Field of the Invention
The present invention relates to a capacitor microphone unique in its shielding structure against electromagnetic wave at a rear acoustic terminal portion.
2. Description of the Related Art
In a capacitor microphone, an impedance converter, typically a field-effect transistor (FET), is disposed nearby a signal output unit of a microphone unit. A signal is output from the microphone with high output impedance of the microphone unit lowered by the impedance converter.
For example, in a unidirectional capacitor microphone, sound waves need to be introduced through a rear acoustic terminal of a microphone unit. Therefore, a casing of the microphone is provided with an opening through which sound waves are introduced to the rear acoustic terminal of the microphone unit. If a signal output unit of the microphone unit is disposed at a position corresponding to the opening, an electromagnetic wave entering the opening from exterior is detected by the impedance converter, i.e., the FET, to be mixed into an output signal of the microphone as noise. Such noise includes hum noise with low frequency attributable to commercial AC source and noise with relatively high frequency attributable to broad cast electromagnetic wave. In addition, generation of noise having extremely high frequency attributable to electromagnetic waves from cell phones that is becoming more frequently used around microphones has recently become a serious problem.
Countermeasures against electromagnetic waves entering the opening through which sound waves are introduced to the rear acoustic terminal in a conventional capacitor microphone have met with limited success. Deficiency of the conventional countermeasures against electromagnetic waves is described below with reference to an exemplary structure of a conventional microphone.
FIG. 4A is an external view exemplary illustrating a typical gooseneck capacitor microphone. FIG. 4B is an external view exemplary illustrating a typical microphone to which a cable is directly connected. In the gooseneck microphone illustrated in FIG. 4A, ends of a flexible pipe 53 is connected to a base end of a casing 51 of a microphone 50 and to a connector 54, respectively. The microphone casing 51 incorporates a capacitor microphone unit. A cable extending from the microphone unit passes through the internal space of the flexible pipe 53 and is electrically connected to a connection terminal of the connector 54. The connector 54 physically fixed to a base (not illustrated) allows the microphone to vertically stand by the flexible pipe 53. The microphone cable can be extended to the exterior via the connector 54 or the cable connector in connection to the connector 54. The microphone casing 51 is provided with an opening 52 through which the rear acoustic terminal of the capacitor microphone unit is communicated with the exterior space.
In the microphone exemplary illustrated in FIG. 4B, a microphone cable 55 extends from the base end of the microphone casing 51 of the capacitor microphone 50 directly or with a connector provided at one end of the microphone cable 55 being connected to thereto. The other end of the microphone cable 55 is provided with a connector 56. An appropriate external device connected to the connector 56 can receive an output signal from the microphone unit. The microphone casing 51 is provided with an opening 52 through which the rear acoustic terminal of the capacitor microphone unit is communicated with the external space.
FIGS. 5A and 5B each exemplary illustrates an internal structure of the capacitor microphone 50. Each of the conventional examples illustrated in FIGS. 5A and 5B includes the microphone casing 51 of a cylindrical shape in which the front end is provided with an endplate and the rear end is opened. The microphone casing 51 incorporates a damper 56 provided along the inner surface of the endplate in the microphone casing 51 and a capacitor microphone unit 57 disposed in the microphone casing 51 in a manner that the damper 56 is pressed against the inner surface of the endplate therewith. The microphone unit 57 is provided with a rear acoustic terminal for the microphone 50 to have unidirectionality. Internal structure of the capacitor microphone unit 57 is described later. The rear acoustic terminal of the capacitor microphone unit 57 is communicated with the external space through multiple openings 52 provided at portions near the front end of the microphone casing 51. An area of the inner surface of the microphone casing 51 including the portions at which the openings 52 are formed is provided with a shield mesh 60 illustrated in FIG. 5C. The shield mesh 60 is made of stainless metal wires having a diameter of about 0.1 mm and is formed into a cylindrical shape that matches the inner surface of the microphone casing 51 to be fixed thereto.
In the structure exemplary illustrated in FIG. 5B, a circuit substrate 58 is fixed inside the microphone casing 51. An output signal terminal of the microphone unit 57 is connected to a certain wiring pattern on the circuit substrate 58. Multiple wires forming the cable 55 are connected to a wiring pattern on the rear end portion of the circuit substrate 58. The cable 55 formed of the multiple wires extends to the exterior through a bush fitted at the opening end of the microphone casing 51.
In the structure exemplary illustrated in FIG. 5A, the wires forming the microphone cable 55 are connected to the circuit substrate of the microphone unit 57. The cable 55 extends to the exterior through the bush fitted at the opening end of the microphone casing 51.
FIG. 6 is an enlarged view of the main part of the capacitor microphone 50, i.e., the capacitor microphone unit 57 and an area therearound. In FIG. 6, the microphone unit 57 includes a unit casing 70 having bottomed cylindrical shape. The microphone unit 57 further includes a diaphragm 64 fixed to a holding ring to be held thereby, a fixed electrode 65 facing the diaphragm 64 with a slight space therebetween, an insulating holder 66 through which pressing force towards the inner bottom surface of the unit casing 70 is applied to the fixed electrode 65 and the holding ring, and a circuit substrate 67 through which the pressing force is applied to the insulating holder 66, all of which are incorporated in the unit casing 70. The opening end of the unit casing 70 is folded in the inner peripheral direction to be a folded portion 71. The folded portion 71 applies pressing force to the periphery of the circuit substrate 67. Thus, the elements incorporated in the unit casing 70 are pressed against the inner bottom surface of the unit casing 70. As a result, the above listed elements are rigidly positioned. A FET 68 serving as an impedance converter is connected to the circuit substrate 67 and is surrounded by a relief hole formed in the insulating holder 66. The flat portion, i.e., the bottom portion of the unit casing 70 is provided with an opening forming a front acoustic terminal. The insulating holder 66 is provided with a rear acoustic terminal 69 through which an air chamber formed between the diaphragm 64 and the fixed electrode 65 is communicated to the exterior and in which an acoustic resistor is provided. The above described structure of the capacitor microphone unit 57 is commonly used and thus is not innovative.
In the structure illustrated in FIG. 6, the circuit substrate 58 is fixed inside the microphone casing 51 as in the structure illustrated in FIG. 5B. The circuit substrates 58 and 67 are perpendicular to each other. A certain circuit pattern on the circuit substrate 58 is soldered to a certain circuit pattern on the circuit substrate 67 on the microphone unit 57 side with one end of the circuit substrate 58 orthogonally abutting contact with the outer side surface of the circuit substrate 67.
The above described conventional capacitor microphone is shielded from electromagnetic waves by covering the openings 52 formed on the microphone casing 51 to achieve unidirectionality from the inside using the shield mesh 60. The shield mesh 60 is made by weaving metallic wires having diameters of about 0.1 mm in the longitudinal and lateral directions as described above. Thus, electrical connection is made at contact portions between the lateral wires and the longitudinal wires. Unfortunately, the shield mesh 60 cannot provide sufficient shielding effect because electrically connected states among the contact points are not uniform. The shield mesh 60 is rolled into a cylinder and is inserted in the microphone casing 51 to be fixed thereto using adhesive. The contact pressure of the shield mesh 60 against the microphone casing 51 may not be high enough to provide sufficient shielding effect.
An output signal of the microphone unit 57 is output directly through the wires as illustrated in FIG. 5A, or is output through the circuit substrate 58 as illustrated in FIG. 5B. The ground portion of the microphone unit 57 is connected to the microphone casing. 51 via the wire or via the circuit substrate 58 and is not directly connected to the microphone casing 51 because a space needs to be provided therebetween. Furthermore, the damper 56 is provided between the front surface of the microphone unit 57 and the inner surface of the endplate serving as the bottom plate of the microphone casing 51. The damper 56 serves as an acoustic resister, a wind screen, and a cushion for the microphone unit 57 against the microphone casing 51.
Typically, in the capacitor microphone unit 57, the circuit substrate 67 on which the FET 68 is mounted fits the opening end of the unit casing 70 incorporating the required elements as illustrated in FIG. 6. The circuit substrate 67 is pressed by the folded portion 71, which is the opening end portion of the unit casing 70 folded inward. Thus, the elements are positioned. In some cases, pressing force towards the circuit substrate 67 of the microphone unit 57 is applied to the wires or the circuit substrate 58 both of which are electrically connected to the circuit substrate 67. If the pressing force is high enough to compress the damper 56, the circuit substrate 67 is separated from the folded portion 71 of the capacitor microphone unit casing 70 as illustrated in FIG. 7. The ground pattern of the circuit substrate 67 is grounded by being in contact with the folded portion 71. Therefore, the above described separation of the circuit substrate 67 from the folded portion 71 of the unit casing 70 hampers the grounded state of the circuit substrate 67 thereby leading to insufficient shielding for the microphone unit 57.
If, on the other hand, the circuit substrate 67 of the microphone unit 57 is applied with pulling force via the wires or the circuit substrate 58, the microphone unit 57 is pulled to the rear direction to increase the distance between the front surface of the microphone unit 57 and the inner surface of the bottom plate of the microphone casing 51. Thus, the front acoustic terminal is widened to cause change in sound quality.
Applicant of the present application has filed Japanese Patent Application Publication 2008-166909 in which an invention is disclosed that can address the problem of the shield for the openings formed in the microphone casing 51, which is one of the above described problems of the conventional capacitor microphone. Specifically, a coil spring is disposed in the microphone casing and the metallic mesh that covers the openings serving as the rear acoustic terminal of the microphone casing from the inside is pressed against the inner wall surface of the microphone casing with the spring force of the coil spring. Thus, the metallic mesh can be surely pressed against the openings of the microphone casing to improve the shielding effect for the openings.
The invention disclosed in Japanese Patent Application Publication 2008-166909 can provide the above described effect. However, the technique, which can provide the shielding effect for the openings of the microphone casing to some extent, still has rooms for improvement because no consideration is made for the shielding effect due to the electrical connection between the casings of microphone unit and the microphone. In addition, no consideration is made for the degradation of the shielding effect and change in sound quality due to pressing force and pulling force applied to the microphone unit, respectively.